1. Field of the Invention
The present invention relates to an image forming device using a transfer roller in order to transfer a toner image formed on a photosensitive body onto paper, and more particularly to such an image forming device equipped with means for controlling the voltage applied to the transfer roller.
2. Description of the Related Art
An image forming device using a general electrophotographic method generally includes a charger for creating a uniform electrical charge on a belt-shaped or drum-shaped photosensitive body, an exposing unit for forming an electrostatic latent image by irradiating light onto the surface of the photosensitive body, a developer for causing toner to adhere to the electrostatic latent image, thereby forming a toner image, and a transfer unit for transferring the toner image formed on the photosensitive body onto paper. The transfer unit is generally a transfer roller. In the developer, there are provided an agitator for agitating the toner inside a toner vessel whilst feeding said toner forwards, and a developing roller for supplying the toner to the photosensitive body. Furthermore, the developing roller is constituted such that a thin layer of toner is formed on the surface of the roller by applying pressure by means of a blade, thereby ensuring that the operation of supplying the toner to the electrostatic latent image formed on the photosensitive body is carried out in a satisfactory manner.
Thereupon, a toner image is formed by causing the toner to adhere to the electrostatic latent image formed on the photosensitive body, and this toner image is transferred to the paper. In the general image forming device described above, means for creating a corona discharge is used for transferring the toner image from the photosensitive body onto the paper, but since the production of ozone is worried about, in many cases, devices are constructed to include a transfer roller which avoids generation of ozone. The transfer roller may include a metal roller shaft and an electrically conductive rubber or plastic layer of a prescribed thickness provided on the shaft. Transfer of a toner image similar to that achieved by a corona discharge is carried out by applying a voltage of a prescribed value to the electrically conductive roller shaft. Since the image forming device produces no ozone due to adoption of the transfer roller, a merit is obtained in that the mechanism for processing exhaust gas from the device can be simplified, and moreover, it is possible to prevent adverse effects on the indoor environment.
In the image transfer means employing the transfer roller, the voltage applied to the transfer roller has a slight effect on the performance of transferring the toner image from the photosensitive body to the printing paper. Therefore, when starting a printing operation, a test voltage is applied in advance, and the transfer voltage is set according to information on the variations in this voltage. However, in cases where a large quantity of printing paper is used consecutively, the temperature inside the device rises as the work progresses, and therefore problems will arise in that the optimum transfer voltage will shift as this temperature increases, and it will not be possible satisfactorily to ensure printing quality. If the initial voltage setting is increased in order to cope with situations where the temperature rises, another problem will then arise in that the photosensitive body and the transfer roller will deteriorate rapidly due to the fact that a higher voltage than required is being applied thereto.
On the other hand, when setting the value of the transfer voltage applied to the transfer roller, the environmental temperature is also taken into account as a control condition, and therefore, within a certain temperature range, the relationship between the current detected when a test voltage is applied and the transfer voltage is determined in accordance with control conditions set in a look-up table in the control unit. However, a problem arises in that in the border regions of the adjacent temperature ranges set in the respective look-up tables, in some cases the transfer voltages do not match and vary considerably with slight change of temperature.
If plural sheets of printing paper are used in a consecutive manner, the transfer roller will continue to rotate in a state of contact with the photosensitive body, even between sheets of paper, and the voltage of a prescribed value will remain applied to the transfer roller. Therefore, in a state where the transfer roller is in contact with the photosensitive body without the printing paper being nipped therebetween, the voltage on the transfer roller acts directly on the photosensitive body, and hence local variations in electric potential may arise on the surface of the photosensitive body, thereby leading to the occurrence of faults in image quality. It can be seen that problems of this kind are especially liable to occur when a higher voltage than required is applied to the transfer roller.